[MikeB]

Here's my take, and I'm open to comments:

13" manifold vacuum at idle in Park would usually indicate a vacuum leak or a cam with lots of overlap. But if your idle is smooth, the cam is probably not bigger than 10-12 degrees above stock. And if turning your idle mixture screws CCW from fully seated doesn't increase vacuum or RPM, then you probably don't have a vacuum leak. But that would typically indicate a rich mixture, caused, for example, by the idle transfer slots being fully exposed.

BTW, 8 degrees max vacuum advance is in the range of what many GM canisters had before emissions controls. There's certainly nothing wrong with it, but once you solve the vacuum problem, it would be nice to get a canister with adjustable spring tension and stop limiter. We eventually started seeing ~20 degrees vacuum (ported) with very little initial + mechanical to reduce idle emissions.

I never would have thought about too much timing chain slack, but it certainly makes sense. You might pick up a couple inches with a new timing set. However, for a stock or mildly modified engine, I suggest you skip the double roller (and potential clearance issues) and get a basic Cloyes timing set. My last few 350s used GM single roller timing sets with the same Renold chain as the GM Performance Parts big blocks. Line up the marks at #1 TDC COMPRESSION STROKE.